This invention concerns an offset printing plate and more particularly it relates to direct use of a silver image obtained by silver complex diffusion transfer process as an offset printing plate.
One type of such offset printing plates, as disclosed in, e.g., U.S. Pat. No. 3,728,114, comprise a light sensitive sheet and an image-receiving sheet and an oleophilic silver image is obtained on the image-receiving sheet by subjecting the light sensitive sheet to an imagewise exposure, then bringing the light sensitive sheet and the image-receiving sheet into superposed relationship and subjecting it to diffusion transfer development. Another type of the printing plates comprise a light sensitive sheet on which an image-receiving layer is directly provided and an oleophilic silver image is obtained by subjecting the sheet to imagewise exposure and then to diffusion transfer development. The light sensitive sheet comprises a support such as a waterproof paper or synthetic resin film which has been treated to render its surface hydrophilic and which carries a photographic emulsion layer mainly composed of silver halide emulsion. When a negative-type emulsion is used as the silver halide emulsion a positive-direct offset printing plate is obtained and when a direct-positive type emulsion is used a negative direct offset printing plate is obtained.
Such direct offset printing plates require printabilities, e.g., good ink receptivity, water retention, surface strength, etc. However, effects of emulsion layers on such printabilities have not yet been fully clarified. Improvements in the emulsion layer are disclosed in Japanese Patent Application Kokai (Laid-Open) No. 55,402/74 and Japanese Patent Application No. 35,767/75, but these are not sufficient.
In order to produce high speed direct offset printing plates, it is necessary to use high speed silver halide emulsion and for this purpose spectral sensitization has been employed. However, when cationic cyanine dyes are used, diffusion transfer development is inhibited to cause reduction in the amount of silver deposited on the image-receiving layer or to cause no deposition of silver. Besides the reduction in the amount of silver, there occurs unevenness on the boundary between the developed and undeveloped portions which is called "edge effect" and printabilities of direct offset printing plates are markedly decreased.